1. Field of the Invention
This invention relates to injection molding machines for the transmission of various molten materials to a mold cavity or cavities. More specifically, this invention relates to a method and apparatus for the insertion of a mixer in the melt stream of an injection molding machine.
2. Summary of the Prior Art
The large number of variables in the injection molding process creates serious challenges to creating a uniform and high quality part. These variables are significantly compounded within multi-cavity molds. Here we have the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot.
Shear induced flow imbalances occur in all multi-cavity molds that use the industry standard multiple cavity xe2x80x9cnaturally balancedxe2x80x9d runner system whereby the shear and thermal history within each mold is thought to be kept equal regardless of which hot-runner path is taken by the molten material as it flows to the mold cavities. These flow imbalances have been found to be significant and may be the largest contributor to product variation in multi-cavity molds.
Despite the geometrical balance, in what has traditionally been referred to as xe2x80x9cnaturally balancedxe2x80x9d runner systems, it has been found that these runner systems can induce a significant variation in the melt conditions delivered to the various cavities within a multi-cavity mold. These variations can include melt temperature, pressure, and material properties. Within a multi-cavity mold, this will result in variations in the size, shape and mechanical properties of the product. Though the effect is most recognized in molds with eight or more cavities, it can create cavity to cavity variations in molds with as few as two cavities.
The flow imbalance in a mold with a geometrically balanced runner is created as a result of shear and thermal variations developed across the melt as it flows through the runner. The melt in the outer region (perimeter) of the runner""s cross-section experiences different shear and temperature conditions than the melt in the center region. As flow is laminar during injection molding, the position of these variations across the melt stream is maintained along the length of the runner branch. When the runner branch is split, the center to perimeter variation becomes a side to side variation after the split. This side to side variation will result in variations in melt conditions from one side to the other of the part molded from the runner branch.
If the runner branches were to split even further, as in a mold with 4 or more cavities, there will exist a different melt in each of the runner branches. This will result in variations in the product created in each mold cavity. It is important to note that as consecutive turns and/or splits of the melt channel occur, the difference in melt temperature and shear history is further amplified. This cumulative effect is clearly recognized in large multi-cavity molds where the runner branches split and turn many times.
In an attempt to reduce this variation, the prior art has been primarily directed at various mixing devices that are located within the runner nozzle which is typically just prior the mold cavity. Examples of this can be found in U.S. Pat. No. 4,965,028 to Manus et al. and U.S. Pat. No. 5,405,258 to Babin.
Mixers at various locations within the injection molding machine are also well known. Examples of mixers in the hot runner manifold include U.S. Pat. No. 5,683,731 to Deardurff et al., European Patent 0293756, U.S. Pat. No. 5,688,462 to Salamon et al. and U.S. Pat. No. 4,848,920 to Heathe et al. (all incorporated herein by reference). An example of mixers installed within the injection unit can be found in U.S. Pat. No. 3,156,013 to Elphee (incorporated herein by reference).
Within the prior art, at least as much as known, there is no retrofit apparatus or method for installation of a mixer in an already existing injection molding machine, specifically in the hot runner manifold. Attempts at alleviating runner imbalance has been directed at correcting the problem within the injection nozzle or further upstream in the machine nozzle or sprue bar.
There exists a need for a mixer apparatus and method that allows for the easy and precise placement of a mixer in the melt stream in an injection molding machine, for example in a hot runner subsystem. Preferably, the mixer should be installed just upstream of where the melt channel splits or divides.
One general objective of the present invention is to provide a mixer apparatus and method that can be easily and precisely placed in an injection molding machine to help alleviate non-homogenity in a melt stream.
Another general object of the present invention is to provide a replaceable mixer insert apparatus and method in an injection molding machine.
Yet another general object of the present invention is to provide a mixer apparatus and method that is completely contained within the hot runner manifold.
The foregoing objects are achieved in one exemplicative embodiment by providing a mixer insert that is sealing placed in a receiving bore, for example, in a hot runner manifold. The mixer insert contains a mixing element that is held in alignment with and communicates with a melt channel. As the non-homogeneous melt flows through the mixing element it is mixed and homogenized thereby reducing melt stream imbalances.
Further objects and advantages of the present invention will appear hereinbelow.